I have not personally sold anything big or small after Simple Delivery has entered our lives, but I believe if an item is not covered by SD scrolling past the Postage options brings you to Custom Postage whereby you can select your own courier ? Can you see that option? You may need a PC / laptop and Chrome not the app or mobile browser to do this.
If not, contact eBay Customer Services, ask them how to proceed.
This is the easiest and quickest way to contact eBay Customer Services, for a Call Back option.
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
Lines open 8 a.m. - 10 p.m. on weekdays
9 a.m. - 6 p.m. on weekends.
Automated agents will be available on chat outside of the above hours.
I strongly recommend contacting CS first thing in the morning as there's more chance of Dublin answering.
@080270phil